The present invention relates to a scanning optical system in which a plurality of beams emitted by multiple light emitting points are dynamically deflected and converged on a scan target surface to form a plurality of beam spots.
By a scanning optical system of this type, a plurality of scan lines can be drawn simultaneously on the scan target surface in one main scanning. By modulating each of the beams according to image information, high-speed imaging is realized.
As a light source for such a scanning optical system, either a single element having a plurality of light emitting points as disclosed in Japanese Patent Provisional Publication No. SHO57-54914 or multiple elements each having a single light emitting point as disclosed in Japanese Patent Provisional Publication No. SHO60-126620 can be used.
However, with either type of light source, if the interval between the beams on the scan target surface is not adjusted precisely (to be equal to moving distance of the scan target surface in one main scan by a reflecting surface divided by the number of beams), the scan line interval on the scan target surface becomes uneven (that is, the interval between scan lines simultaneously drawn becomes inconsistent with the interval between scan lines drawn by different main scans) as described in the Publication No. SHO57-54914, and in such a case, the quality of printed images is deteriorated. Therefore, the interval between the beams on the scan target surface has to be adjusted correctly.
When the multiple light emitting elements are used as in the scanning optical system of the Publication No. SHO60-126620, the beam interval on the scan target surface can be corrected by changing the positional relationship among the elements. However, the movement of the element is magnified by transverse magnification of the whole optical system, resulting in a shift of the beam on the scan target surface (shift in the auxiliary scanning direction which is orthogonal to the direction of the scan lines). Therefore, the adjustment of each light emitting element, requiring extremely high accuracy, can not be done easily by a nonexpert.
Meanwhile, when a single light emitting element having a plurality of light emitting points is used as in the scanning optical system of the Publication No. SHO57-54914, the interval between the light emitting points may be fixed at its design interval. However, if the interval between the beams deviates from a design interval due to magnification error of the whole optical system caused in the manufacturing process (e.g., an error in the relationship of the light emitting element with other optical elements), it is impossible to correct the beam interval by adjusting the interval between the light emitting points of the element. Therefore, in such a case, the beam interval on the scan target surface is corrected by adjusting the magnification of the whole optical system by placing an afocal anamorphic zoom lens system (composed of three lens groups as described in the Publication No. SHO57-54914) between a collimating lens and a cylindrical lens. However, such an afocal anamorphic zoom lens system, which is originally unnecessary for the function of the scanning optical system, drives up the cost of the system.
Incidentally, when such a single light emitting element having a plurality of light emitting points is employed, it is also possible to adjust the interval between the scan lines on the scan target surface by rotating the light emitting element in a plane containing the light emitting points. However, the rotation of the light emitting element naturally causes a change in the interval between the light emitting points in the main scanning direction, by which extra adjustment such as readjustment of emission timing of each light emitting point becomes necessary. Further, the rotation of the light emitting element also causes rotation of the far field pattern of the beams, accompanied by problems such as a change in coupling efficiency.